1. Field of the Invention
The present invention relates to a method for manufacturing a molded transformer with a wound core.
2. Description of the Related Art
In conventional molded transformers, coils impregnated with a resin are combined with a laminated core after the resin is set. These days, however, the molded transformers are expected to have reduced iron loss as well as reduced size and weight. To meet these requirements, for example, a wound core permitting less iron loss and lower exciting current is tried in place of the laminated core, as in the case of an oil-immersed transformer.
In manufacturing such a molded transformer with a wound core, however, if resin-molded coils are combined with the wound coil, it is difficult to prevent rusting of the leg portions of the wound core, which are located within the coil, and generation of noise due to vibration.
To cope with this difficulty, a method may be adopted in which a wound core-coil assembly consisting of a wound core and coils wound therearound is immersed in varnish or resin in a reservoir so that the varnish or resin adheres to the assembly, and the assembly is then taken out from the reservoir and put into a drying furnace to cure the varnish or resin. In this method, however, the varnish or resin in a liquid phase drips when the wound core-coil assembly is removed from the reservoir. Thus, it is impossible to obtain a uniform resin layer. Moreover, voids may remain in the varnish or resin, causing defective insulation or lowering adhesive strength. Also, the thermal conductivity may be lowered, resulting in deterioration of the heat radiation characteristic.
As with small molded transformers for a measuring instrument, etc., a method may be proposed in which a one-coil assembly consisting of a wound core and coils wound therearound is put into a die, and a resin is impregnated into the assembly and thermally set to form an integral molding. When this method is applied to the distribution transformer, the resin thickness is increased, and the interior of the transformer is heated at a high temperature when the coils are energized. The injected resin cracks due to the difference in thermal expansion coefficients between the wound core and the resin, as well as its increased thickness, thus lowering its insulating capability. In order to prevent the cracking, a shock absorbing member may be provided around the wound core. The use of such a member may, however, result in a complicated structure of the transformer. Moreover, since the coil generally requires cooling ducts, the manufacture of the cooling ducts necessitates the use of a die with a complicated construction.
Furthermore, a method is disclosed in Japanese Patent Publication No. 25127/77 in which a molded wound core is divided into two and the outer surface is coated with rust-resistant paint with its divided ends contacting together. The divided ends are coated with an adhesive agent, and then the wound core is assembled into coils. Thereafter the divided ends of the core are joined together.
This method is subject to the drawback that the adhesive agent on the joined ends of the wound core deteriorates over time, and this results in noise during the operation of the transformer.